Mechanical joint with switchable, rotation-constraining clutch

ABSTRACT

The current document is directed to a two-way, by-passable, overrunning clutch incorporated within a mechanical prosthetic knee that provides functionality similar to a biological knee or incorporated in another articulated device, such as a robotic or orthotic articulated device or member. The currently disclosed two-way, by-passable, overrunning clutch allows for two-way free rotation when disabled, but, when enabled, prevents rotation in one direction while allowing free rotation in the other direction. In the mechanical prosthetic knee, the two-way, by-passable, overrunning clutch is enabled by application of a mechanical force and disabled by removal of the mechanical force.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to PCT Application No.PCT/US2016/058948; filed Oct. 26, 2016, which claims the benefit ofProvisional Application No. 62/246,485, filed Oct. 26, 2015.

TECHNICAL FIELD

The present disclosure is directed to artificial joints used inprosthetic, exoskeletal, orthotic, and/or robotic devices, and, inparticular, to a two-way, by-passable, overrunning clutch that isincorporated into prosthetic joints to provide for a switchableconstraint to rotation.

BACKGROUND

The design and manufacturing of prosthetic limbs and joints is a largeand important industry. Prosthetics are used to address limb-lossinjuries from military conflicts, transportation accidents, and manydifferent types of diseases and pathologies, including a currentlyrising incidence of diabetes. Currently, large design and developmentefforts are directed towards sophisticated electromechanical prostheticscontrolled by one or more microprocessors, with some designs directed toat least partially controlling motion and operation of a prosthetic limbthrough sensing and responding to nervous activity within a patients'bodies. Many of these complex and expensive prosthetics providesimulated natural motion and biological-limb-like operationalcharacteristics, but they are associated with many disadvantages. Afirst disadvantage is cost. For many people, even in developedcountries, an artificial limb costing many tens of thousands of dollarsto hundreds of thousands of dollars is beyond reach. In many developingcountries, including poorer countries wracked by military conflicts andresidual dangers, such as land mines and undetected unexploded bombs,modern, sophisticated electromechanical prosthetics are far tooexpensive even for relatively wealthy inhabitants. The sophisticatedelectromechanical prosthetics also have significant shortcomings,including difficulties in providing reliable power sources, steeplearning curves, physical-fitness requirements, and training overheadsassociated with use of such prosthetics, many types of failure modesthat generally accompany complex designs and implementations, includinglack of water resistance and vulnerability to damaging environmentalagents, and frequent maintenance and repair overheads. For thesereasons, there remains great interest in developing new mechanicalprosthetics that are simple to manufacture, relatively inexpensive,robust, and reliable.

SUMMARY

The current document is directed to a two-way, by-passable, overrunningclutch incorporated within a mechanical prosthetic knee that providesfunctionality similar to a biological knee or incorporated in anotherarticulated device, such as a robotic or orthotic articulated device ormember. The currently disclosed two-way, by-passable, overrunning clutchallows for two-way free rotation when disabled, but, when enabled,prevents rotation in one direction while allowing free rotation in theother direction. In the mechanical prosthetic knee, the two-way,by-passable, overrunning clutch is enabled by application of amechanical force and disabled by removal of the mechanical force.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the human gait cycle.

FIG. 2 provides an exploded diagram of the currently disclosed two-way,by-passable, overrunning mechanical clutch that includes adual-wrap-spring clutch element and that is incorporated in mechanicalprosthetic knee joint.

FIGS. 3A-D exaggeratingly illustrate the clutching operation of thedual-wrap-spring clutch element.

FIG. 4 shows the assembled two-way, by-passable overrunning clutchmechanism of the currently disclosed prosthetic knee.

FIG. 5 shows a prosthetic leg that incorporates the two-way, by-passableoverrunning clutch described above.

FIGS. 6A-C illustrate operation of the prosthetic leg shown in FIG. 5.

FIG. 7 shows an orthotic exoskeleton-like device that may be worn topromote healing of an injured leg or to provide increased functionalityto a damaged limb.

FIGS. 8A-B illustrate dual-mechanical-clutch assemblies that may beincorporated into more complex prosthetic joints and robot assemblies.

DETAILED DESCRIPTION

FIG. 1 illustrates the human gait cycle. FIG. 1 shows the gait cycle forthe right leg of a person walking forward, in a left-to-right direction,along a horizontal surface 102. In FIG. 1, the right leg is abstractlyrepresented by a rotating hip joint 104, an upper-leg shaft 106, arotating knee joint 108, a lower-leg shaft 110, a rotating ankle joint112, and a foot 114. In the first position 116 shown in FIG. 1, the leghas been thrust forward, during walking, with the heel of the foot 114just making contact with the horizontal surface 102. As the personcontinues walking in the forward direction, the person is propelledforward largely by exertion of muscles in the other leg along with aforward shift in the person's center of mass, with the knee jointrotating to allow the person's body to travel forward while the footremains securely positioned, by friction, on the horizontal surface.This is illustrated in the second position 118 shown in FIG. 1. Inposition 120, the person's body has moved further forward, straighteningthe leg. In position 122, the knee joint continues to rotate, allowingthe center of mass of the person's body to continue forward, past thefoot and knee. Rotation of the knee joint continues further to position124, where the ankle joint and knee joint both rotate to allow theperson's heel to rise above the horizontal surface and to allow muscleforce of the leg to propel the person forward. In position 126, theperson's left leg is moving from position 116 to 118, allowing thedisplayed right leg to become unweighted and the foot to be raised to aposition in which it does not contact the horizontal surface. Thisallows the knee to rotate in an opposite direction, in positions 128-130so that the lower leg swings forward in order to make contact with theground. Note that position 130 is equivalent to position 116, completingthe gait cycle.

The gait cycle can thus be described generally as having a stance phase132 and a swing phase 134. In the stance phase, the knee joint needs toallow the lower leg to rotate in clockwise direction, from theperspective of FIG. 1, in order to straighten from position 118 toposition 120. However, the knee must resist rotation in acounter-clockwise direction to prevent the leg from buckling, due to thecenter of mass of the person's body being located behind the position ofthe knee, and a resulting fall. By contrast, during the swing phase 134,the knee continues to allow rotation of the lower leg in the clockwisedirection but then allows the lower leg to rotate in thecounter-clockwise direction in order to facilitate thrusting of the legforward to begin a next stance phase. In general, during the stancephase, the leg is weighted due to the gravitational force of theperson's body pushing down through the leg structure while, in the swingphase 134, the leg is unweighted.

For transfemoral amputees, a prosthetic knee joint is generally used topermit walking and other types of locomotion and movement. Prostheticknees generally attempt to provide rotational constraints during thestance phase, to prevent buckling of the knee, while allowing the kneeto swing relatively freely during the swing phase. While a certain levelof rotational constraints during the swing phase may be overcome by anamputee by altering the gait cycle, such as a modified gait cyclereferred to as “peg-leg walking,” failure of a prosthetic knee toprovide for only one-way rotation during the stance phase would renderthe prosthetic knee unsuitable and dangerous.

As discussed above, prosthetic knees can be broadly classified as eithermicroprocessor-controlled, electromechanical prosthetic knees, or aspurely mechanical knees without microprocessors and generally withoutdigital electronics. Various different types of mechanical knees usehydraulics, pneumatics, friction braking, and/or geometricalinterferences of linkages in order to prevent unconstrained kneerotation when the leg is weighted, in the stance phase.Electromechanical knees may use similar mechanical components, butgenerally employ software-controlled microprocessors and otherdigital-electronic components to control operation of the prostheticknee.

In the case of conventional, currently available mechanical knees, theamputee learns to control the knee with his/her remaining muscles andexecute certain techniques during gait to engage knee stability duringstance. Unless the user puts the knee in the correct position, the kneeis unstable. This takes a great deal of training and expertise andimposes an extra mental workload on the amputee, which, in turn, meansmore attention and energy expended during walking. Nevertheless,conventional mechanical knees remain widely used because they aregenerally more durable and less expensive than microprocessor knees.

Microprocessor-controlled knees, on the other hand, monitor varioussensors, determine a user's position in the gait cycle, andautomatically control engagement of stability control for the user. Infact, such knees remain in stability mode most of the time. This resultsin greater efficiency in gait and decreased mental workload. However,microprocessor-controlled knees are significantly more expensive,require a power source (typically a battery), and often involvesignificant tuning and maintenance to function properly.

The current document discloses a new mechanical prosthetic-knee jointthat provides the desired constraints on the relative rotation of thelower leg and upper leg when the leg is weighted, during the stancephase, but also provides relatively free rotation of the lower leg withrespect to the upper leg, in both directions, when the leg is unweightedduring the swing phase. The currently disclosed mechanical prostheticknee joint is, in a described implementation, a fully mechanical,non-microprocessor-based knee prosthetic. However, in alternativeimplementations, electronics and control logic, including either or bothof state machines and microprocessors, can be incorporated into aprosthetic mechanical knee joint that employs the mechanical componentsof the currently disclosed mechanical prosthetic knee. In other words,while the currently disclosed mechanical prosthetic knee is adequate forpurely mechanical implementations, the currently disclosed mechanicalcomponents may additionally be used in more complex, electromechanicalprosthetic knees.

The mechanical constrained-rotation mechanism used in the currentlydisclosed prosthetic knee is a type of two-way, by-passable, overrunningmechanical clutch. This mechanical clutch is based on a single-piece,dual-wrap-spring clutch element (“WSC”). FIG. 2 provides an explodeddiagram of the currently disclosed two-way, by-passable, overrunningmechanical clutch that includes a dual-wrap-spring clutch element andthat is incorporated in mechanical prosthetic knee joint. As shown inFIG. 2, the mechanical prosthetic knee joint includes the WSC 202, twoarbors 204 and 206, two arbor sleeves 208 and 210, two clutch pins 212and 214, a yoke 216, a cam-like force adjustor and dimensionalconstraint 218, a torque transmission pin 220, a torque-transmission-pinspring 222, two flexible linkages 224 and 226, and a lower-leg block228.

In FIG. 2, were the mechanical prosthetic knee to include a kneecap, itwould be facing outward in the direction of arrow 230. The back of theknee would be facing in the direction of arrow 232. The lower-leg block228 is rigidly affixed to a lower-prosthetic leg assembly, andtorque-transmission pin 220 rotationally couples the lower-leg block 228to the WSC, while the yoke 216 and arbors 204 and 206 are rigidlyaffixed to an upper-prosthetic-leg assembly. The WSC 202 rotates, alongwith the torque-transmission pin 220 and lower-leg block 228, relativeto the arbors 204 and 206 and yoke 216. The two-way by-passableoverrunning clutch mechanism operates to prevent rotation of thelower-leg block 228 and torque-transmission pin 220 in acounter-clockwise direction (in the perspective of a viewer looking intothe figure) with respect to the yoke 216 and arbors 204 and 206 when theleg is weighted, in the stance phase, but allows rotation of thelower-leg block 228 and torque-transmission pin in the clockwisedirection. In the swing phase, when the leg is unweighted, the lower-legblock 228 and torque-transmission pin 220 can freely rotate in both thecounter-clockwise and clockwise directions with respect to the yoke 216and arbors 204 and 206.

The WSC provides friction-based clutching. The WSC 202 includes twooutward-spiraling helical coils 234 and 236 with flattened coils thatprovide a cylindrical surface complementary to the arbor shafts,discussed below. When the lower-leg block 228 and torque-transmissionpin 220, which is affixed through an aperture in a lower portion of theWSC central band 238, rotates outward in the direction of arrow 230, ina clockwise direction with respect to the yoke 216 and arbors 204 and206, the outward-spiraling coils rotate freely over the surface of thecylindrical arbor shafts 240 and 242. As the WSC rotates with respect toarbors 204 and 206, the outward-spiraling helices appear to be travelinghorizontally inwards, towards the central WSC band 238, like the threadsof a screw. However, when the lower-leg block 228 andtorque-transmission pin 220 are pushed from the front backward, in thedirection of arrow 232, in an attempt to rotate the lower-leg block 228and torque-transmission pin 220 in a counter-clockwise direction withrespect to the yoke 216 and arbors 204 and 206, the outwardly spiralinghelices 234 and 236 compress down tightly against the surface of thearbor shafts 240 and 242, preventing rotation. Were these outwardlyspiraling helices able to rotate along with the lower-leg block andtorque-transmission pin in the counter-clockwise direction, the heliceswould appear to travel outward, in the horizontal direction, towards thedistal ends of the arbors 204 and 206.

The arbor sleeves 208 and 210 fit over the arbors 204 and 206 in orderto ride above the outwardly spiraling helices of the WSC, with hexagonalnut-like ends 244 and 246 of the WSC mating within complementaryhexagonal end fittings 248 of the arbor sleeves 208 and 210. Flexiblelinkages 224 and 226 are affixed to the lower-leg block 228 via pinsthrough apertures 250-253 and are affixed, via apertures 254-257, toarbor sleeves 208 and 210. When the leg is weighted, the yoke 216, arborsleeves 208 and 210, arbors 204 and 206, and WSC 202 are forceddownward, against torque-transmission pin spring 222 towards thelower-leg block 228. This downward translation of the yoke 216, arborsleeves 208 and 210, and arbors 204 and 206 result in an upward forcetransmitted through flexible linkages 234 and 226 to the arbor sleeves210 and 208, causing the arbor sleeves to rotate in a clockwisedirection with respect to arbors 206 and 204 and yoke 216. This rotatesthe ends of the WSC 244 and 246 in a clockwise direction which which maytighten the outwardly spiraling helices against the arbor shafts 240 and242, allowing the clutching action and preventing rotation of thelower-leg block 228 and torque-transmission pin 220 in acounter-clockwise direction with respect to the yoke 216 and arbors 204and 206. When the leg is unweighted, the torque-transmission-pin spring222 pushes the WSC 220 upward with respect to the lower-leg block,resulting in a downward force applied by the flexible linkages 224 and226 to the arbor sleeves 208 and 210. This results in acounter-clockwise rotation of the arbor sleeves with respect to thearbor 204 and yoke 216, which applies a counter-clockwise rotation tothe outwardly spiraling WSC helices 234 and 236, resulting in expandingthe diameter of the outwardly spiraling helix cylinders and allowing thehelical cylinders to rotate with respect to the arbor shafts.

The tines 260, 262, 264, and 266, two on each side of yoke 216, fit downover the vertical end bars 270 and 272 of the arbors 204 and 206 in thecompleted assembly, and are affixed to the arbors via clutch pins 212and 214, which also pass through apertures 274 and 276 at the ends ofthe cam-like force adjustor and dimensional constraint 218. The cam-likeforce adjustor and dimensional constraint therefore holds the arbors ata fixed horizontal distance from one another, to resist outward pressurefrom the outwardly-spiraling helical coils 234 and 236. In addition, thecylindrical cam 278 includes a partial shaft aperture 280 within whichthe torque-transmission pin slides. As the prosthetic knee jointstraightens, additional frictional force is applied by the cam-likeforce adjustor and dimensional constraint to the torque-transmission pinwhich, in turn, increases the effect of weight applied to the leg inenabling the two-way, by-passable overrunning clutch mechanism. In asitting position, an amputee can overcome the clutch operation morereadily than in a standing position, allowing for transition from thesitting position to the standing position.

The two-way, by-passable, overrunning mechanical clutch is mechanicallysymmetrical and fault tolerant. A single wrap spring could, uponmechanical failure, lead to accidents. By using two wrap springs, asingle-coil failure does not result in failure of rotational constraint,but is noticeable to an amputee, alerting the amputee that repair isneeded. Furthermore, the dual-wrap-spring configuration providessymmetrical distribution of loading forces through the two-way,by-passable, overrunning mechanical clutch, relieving unbalanced stressand potential failure modes.

FIGS. 3A-D exaggeratingly illustrate the clutching operation of thedual-wrap-spring clutch element. FIGS. 3A-B show the WSC in an open,disengaged configuration. In this configuration, the diameter of theflattened, outwardly spiraling helical coils 234 and 236 increases fromthe central WSC band 238 outward towards ends 302 and 304. FIG. 3Bprovides a different perspective of the disengaged WSC clutchconfiguration. As discussed above, when the leg is weighted, theflexible linkages (224 and 226 in FIG. 2) impart a clockwise rotation tothe arbor sleeves 208 and 210, tightening the helical coils of the WSCagainst the arbor shafts, as exaggeratingly shown in FIGS. 3C and 3D.The diameter of the outwardly spiraling helical coils 234 and 236decreases from the central WSC band 238 towards the ends 302 and 304.FIG. 3D shows the tightened helical coils from a different perspective.However, in actual operation, the changes in diameter along the lengthsof the outwardly spiraling helical coils of the WSC are visuallyimperceptible.

FIG. 4 shows the assembled two-way, by-passable overrunning clutchmechanism of the currently disclosed prosthetic knee. In this view, theyoke 216 is securely mounted over the vertical bars 272 of the arbors206, only one of which is visible in FIG. 4. The two flexible linkages224 and 226 are shown mounted to the lower-leg block 228 and the arborsleeves 208 and 210 via pins 402-405. The end of clutch pin 214, 408, isseen at the edge of the yoke 216 and the end 406 of the cam-like forceadjustor and dimensional constraint 218 in FIG. 2 is seen on the outersurface of the vertical bar 272 of the arbor 206.

FIG. 5 shows a prosthetic leg that incorporates the two-way, by-passableoverrunning clutch described above. The two-way, by-passable overrunningmechanical clutch 502 is shown attached to a lower-leg shank 504 and toan upper-leg truncated-limb sleeve 506. FIGS. 6A-C illustrate operationof the prosthetic leg shown in FIG. 5. When the amputee is sitting, thetruncated-limb sleeve 506 is orientated approximately orthogonally tothe lower-leg shaft 504. The two-way, by-passable overrunning clutchmechanism 502 is unweighted, and thus the clutch mechanism is disabled.As shown in FIG. 6B, as the amputee begins to stand, the two-way,by-passable overrunning clutch 502 is partially weighted, resulting inan enabling of the clutch and resistance to rotation of thetruncated-limb sleeve 506 back downward to the position shown in FIG.6A. In FIG. 6C, the amputee is standing on the prosthetic limb, as aresult of which the two-way, by-passable overrunning clutch 502 is fullyenabled, preventing rotation of the truncated-limb sleeve 506 backwardwith respect to the lower-leg shank 504 but allowing free rotation ofthe lower-leg shank 504 in a counter clockwise direction relative to thetruncated-limb sleeve 506.

FIG. 7 shows an orthotic exoskeleton-like device that may be worn topromote healing of an injured leg or to provide increased functionalityto a damaged limb. The orthotic 702 incorporates several two-way,by-passable overrunning mechanical clutch mechanisms 704 and 706 toprovide for free rotation of the lower-leg portion of the orthotic 708with respect to the upper portion of the orthotic 710 when the orthoticis unweighted but preventing rotation of the lower-leg portion 708backward, in a clockwise direction, with respect to the upper-legportion 710 when the orthotic is weighted.

FIGS. 8A-B illustrate dual-mechanical-clutch assemblies that may beincorporated into more complex prosthetic joints and robot assemblies.In the dual-mechanical-clutch assembly shown in FIG. 8A 802, twotwo-way, by-passable overrunning clutch mechanisms 804-805 are joinedtogether by a fixed member 806 joined to, or incorporating, the centralbands of the two WSCs within the two two-way, by-passable mechanicalclutch mechanisms 804-805. FIG. 8B shows an alternative dual-clutchmechanism in which the two two-way, by-passable overrunning clutchmechanisms 810 and 812 are orthogonally disposed to one another throughconnecting member 814.

Although the present invention has been described in terms of particularembodiments, it is not intended that the invention be limited to theseembodiments. Modifications within the spirit of the invention will beapparent to those skilled in the art. For example, the variouscomponents of the two-way, by-passable overrunning clutch mechanism maybe made from a variety of different materials, including metals,composites, plastics, and other materials. They may be molded, cast,machined, or even printed by 3D printing. The flexible links, describedabove, may be flexible and fixed at either end, as described, or mayalternatively be rigid and pivot about their attachment points. Theinterface between the WSC central body and the shank may be of any formthat resists or prevents relative rotation between the two but allowsrelative longitudinal translation. Examples include but are not limitedto a Sarrus linkage, multiple linear shafts, shafts of differentcross-section, or a flexible beam. The control methodology may beinverted, with upper leg movement relative to the WSC controlling thebraking action of the WSC. Control sleeves may be rotated by mechanicalmeans as previously described, or by induced electromagnetic fields,pneumatic systems, or hydraulic systems. Pneumatic, hydraulic, orelectromagnetic systems could provide the adjustable spring forcebetween the WSC and shank. The non-microprocessor mechanical prostheticknee joint may be coupled with additional WSC units of the same orsimilar type to create a joint system that has multiple axes of rotation(e.g., polycentric knees) as in FIG. 6A. Further, the axes of rotationmay lie in any arbitrary plane(s), thus providing motion in differentdirections (e.g., shoulder joint, hip joints, etc.). The currentlydisclosed two-way, by-passable, overrunning clutch may be used as ajoint in the limb(s) of a robot. In particular, the knee joint systemenables a walking robot to walk more efficiently, with greaterstability, and with increased ability to recover from falls. Thenon-microprocessor mechanical prosthetic knee joint can be split intotwo parts through the WSC central body. This enables mounting of thejoint system on either side of an intact knee or other joint, to serveas an integral component of an exoskeleton or orthotic system. In suchcase a user need not be an amputee, but instead may use thenon-microprocessor mechanical prosthetic knee joint to augment orsupplement their existing strength, gait, posture, and/or range ofmotion. As mentioned above, the two-way, by-passable overrunning clutchmechanism may additionally be incorporated in a variety ofelectromechanical and higher-end prosthetics.

It is appreciated that the previous description of the disclosedembodiments is provided to enable any person skilled in the art to makeor use the present disclosure. Various modifications to theseembodiments will be readily apparent to those skilled in the art, andthe generic principles defined herein may be applied to otherembodiments without departing from the spirit or scope of thedisclosure. Thus, the present disclosure is not intended to be limitedto the embodiments shown herein but is to be accorded the widest scopeconsistent with the principles and novel features disclosed herein.

The invention claimed is:
 1. A mechanical joint comprising: a lower-legblock; and a yoke assembly having a clutch-element member that has acentral cylindrical band and that mechanically prevents rotation of thelower-leg block relative to the yoke assembly in one rotationaldirection when the mechanical joint is weighted and that mechanicallypermits free rotation of the lower-leg block relative to the yokeassembly when the mechanical joint is unweighted, a yoke-arborsubassembly that includes a yoke with a first arbor mount and a secondarbor mount, a first arbor with a cylindrical shaft mounted to the firstarbor mount, a second arbor with a cylindrical shaft mounted to thesecond arbor mount, a first coil attached to a first edge of the centralcylindrical band of the clutch-element member and spiraling away fromthe central cylindrical band, the first coil rotatably mounted to thefirst and a second coil attached to a second edge of the centralcylindrical band clutch-element member and spiraling away from thecentral cylindrical band in a direction opposite from that of the firstcoil, the second coil rotatably mounted to the second arbor.
 2. Themechanical joint of claim 1 wherein the first and second coils clampdown on the first and second cylindrical shafts of the first and secondarbors, respectively, when the mechanical joint is weighted to preventrotation of the lower-leg block relative to the yoke assembly in the onerotational direction.
 3. The mechanical joint of claim 1 wherein theyoke includes a structural member, the first arbor mount, and the secondarbor mount; wherein each of the first and second arbors includesmounting members complementary to the first arbor mount and the secondarbor mount, respectively: wherein, when the mounting member of thefirst arbor is fitted to the first arbor mount and is additionallysecured to the first arbor mount by a first securing member, the firstarbor is rigidly mounted to the yoke; and wherein, when the mountingmember of the second arbor is fitted to the second arbor mount and isadditionally secured to the second arbor mount by a second securingmember, the second arbor is rigidly mounted to the yoke.
 4. Themechanical joint of claim 1 wherein the yoke assembly further comprises:a first cylindrical arbor sleeve that is rotatably mounted to thecylindrical shaft of the first arbor, with the first coil lying betweenan inner surface of the first cylindrical arbor sleeve and a surface ofthe cylindrical shaft, an end member of the first coil mated to acomplementary member of the first cylindrical arbor sleeve; and a secondcylindrical arbor sleeve that is rotatably mounted to the cylindricalshaft of the second arbor, with the second coil lying between an innersurface of the second cylindrical arbor sleeve and a surface of thecylindrical shaft, an end member of the second coil mated to acomplementary member of the second cylindrical arbor sleeve.
 5. Themechanical joint of claim 4 wherein, when the mechanical joint isunweighted, the first and second cylindrical arbor sleeves are rotatedwith respect to the first and second arbors, respectively, imparting atwisting force to the first and second coils, respectively, whichresults in loosening the interaction between the coils and the arborshafts, allowing free rotation of the lower-leg block with respect tothe yoke assembly.
 6. The mechanical joint of claim 4 wherein linkingmembers that link the lower-leg block to the cylindrical arbor sleevesprovide a rotational force to the cylindrical arbor sleeves when thelower-leg block moves away from the clutch element.
 7. The mechanicaljoint of claim 4 wherein the end members of the first and second coilsare cross sections of a regular prism that fit into the complementarymembers of the first and second cylindrical arbor sleeve.
 8. Themechanical joint of claim 4 wherein a torque-transmission pin is springmounted to the lower-leg block and passes through an aperture in thecentral cylindrical band; and wherein the yoke assembly furthercomprises a cylindrical cam that includes a partial shaft aperturewithin which the torque-transmission pin slides.
 9. The mechanical jointof claim 8 wherein the torque-transmission pin rotationally couples thelower-leg block to the clutch element.
 10. The mechanical joint of claim8 wherein the cylindrical cam is secured to the yoke so that the depthof shaft aperture varies as the torque-transmission pin and lower-legblock rotate with respect to the yoke assembly.